The present invention is directed to a method for increasing the yield of light olefins during the conversion of oxygenates to olefins by converting heavy hydrocarbons in the product to light olefins either by (a) recycling the heavy hydrocarbons to the primary reactor after removing the light olefins, or (b) converting the heavy hydrocarbons to light olefins in a separate auxiliary reactor.
Light olefins (defined herein as xe2x80x9cethylene and propylenexe2x80x9d) serve as feeds for the production of numerous chemicals. Light olefins traditionally are produced by petroleum cracking. Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
Alternative feedstocks for the production of light olefins are oxygenates, such as alcohols, particularly methanol, dimethyl ether, and ethanol. Alcohols may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for olefin production.
Because light olefins are the most sought after products of such a reaction, a continuing need exists for new methods to increase the yield of light olefin products and reduce the yield of unwanted products, such as xe2x80x9cheavyxe2x80x9d hydrocarbons having molecular weights heavier than propane.
The present invention provides a method for increasing light olefin yield during conversion of oxygenates to olefins. The method comprises: contacting an oxygenate feed in a primary reactor with a non-zeolitic molecular sieve catalyst under first conditions effective to produce a first product comprising light olefins; separating the first product into light olefins and a heavy hydrocarbon fraction; feeding the heavy hydrocarbon fraction either back to the primary reactor or to a separate auxiliary reactor; and, subjecting the heavy hydrocarbon fraction to second conditions effective to convert at least a portion of the heavy hydrocarbons to light olefins.
In the conversion of oxygenates to light olefins (defined herein as ethylene and propylene), it is desirable to maximize the yield of light olefins. The present invention maximizes the yield of light olefins by converting the xe2x80x9cheavy hydrocarbon fractionxe2x80x9d of the reaction product to olefins. The xe2x80x9cheavy hydrocarbon fractionxe2x80x9d is defined herein as the fraction containing hydrocarbons having a molecular weight greater than propane. The conversion of heavy hydrocarbons to light olefins is accomplished either by (a) returning all or a part of the heavy hydrocarbon fraction to the primary reactor, where the heavy hydrocarbons are converted to light olefins along with additional oxygenate feed, or (b) conveying the heavy hydrocarbon fraction to a separate auxiliary reactor where the heavy hydrocarbons are converted to light olefins.
Molecular sieve catalysts that are suitable for use in the primary reactor are non-zeolitic catalysts, which include, but are not necessarily limited to silicoaluminophosphates (xe2x80x9cSAPO""sxe2x80x9d). SAPO""s have a three-dimensional microporous crystal framework of PO2+, AlO2xe2x88x92, and SiO2 tetrahedral units. Preferred SAPO""s for use in the primary reactor are xe2x80x9csmallxe2x80x9d and xe2x80x9cmediumxe2x80x9d pore SAPO""s. xe2x80x9cSmall porexe2x80x9d molecular sieve catalysts are defined as catalysts with pores having a diameter of less than about 5.0 Angstroms. xe2x80x9cMedium porexe2x80x9d molecular sieve catalysts are defined as catalysts with pores having a diameter in the range of from about 5 to about 10 Angstroms.
Suitable SAPO""s for use in the invention include, but are not necessarily limited to SAPO-11, SAPO-44, SAPO-34, SAPO-17, and SAPO-18. A preferred SAPO is SAPO-34, which may be synthesized according to U.S. Pat. No. 4,440,871, incorporated herein by reference, and Zeolites, Vol. 17, pp. 512-522 (1996), incorporated herein by reference.
SAPO""s with added substituents also may be useful in the present invention. These substituted SAPO""s form a class of molecular sieves known as xe2x80x9cMeAPSO""s.xe2x80x9d Suitable substituents include, but are not necessarily limited to nickel, cobalt, strontium, barium, and calcium.
Any molecular sieve catalyst capable of converting hydrocarbons with 4 or more carbon atoms into light olefins may be used in an auxiliary reactor. Preferred molecular sieve catalysts for the auxiliary reactor are zeolites.
Structural types of zeolites that are suitable for use in the auxiliary reactor with varying levels of effectiveness include, but are not necessarily limited to AEI, AFT, APC, ATN, ATT, ATV, AWW, BIK, CAS, CHA, CHI, DAC, DDR, EDI, ERI, GOO, KFI, LEV, LOV, LTA, MON, PAU, PHI, RHO, ROG, and THO and substituted examples of these structural types, as described in W. M. Meier and D. H. Olsen, Atlas of Zeolite Structural Types (Butterworth Heineman-3rd ed. 1997), incorporated herein by reference. Structural types of medium pore molecular sieves useful in the present invention include, but are not necessarily limited to, MFI, MEL, MTW, EUO, MTT, HEU, FER, AFO, AEL, TON, and substituted examples of these structural types, as described in the Atlas of Zeolite Types, previously incorporated herein by reference. A preferred zeolite for the auxiliary reactor is ZSM-5.
The process for converting oxygenates to olefins employs an organic starting material (feedstock) preferably comprising xe2x80x9coxygenates.xe2x80x9d As used herein, the term xe2x80x9coxygenatesxe2x80x9d is defined to include, but is not necessarily limited to aliphatic alcohols, ethers, carbonyl compounds (aldehydes, ketones, carboxylic acids, carbonates, and the like), and also compounds containing hetero-atoms, such as, halides, mercaptans, sulfides, amines, and mixtures thereof. The aliphatic moiety preferably should contain in the range of from about 1 to about 10 carbon atoms and more preferably in the range of from about 1 to about 4 carbon atoms. Representative oxygenates include, but are not necessarily limited to, lower straight chain or branched aliphatic alcohols, their unsaturated counterparts, and their nitrogen, halogen and sulfur analogues. Examples of suitable compounds include, but are not necessarily limited to: methanol; ethanol; n-propanol; isopropanol; C4-C10 alcohols; methyl ethyl ether; dimethyl ether; diethyl ether; di-isopropyl ether; methyl mercaptan; methyl sulfide; methyl amine; ethyl mercaptan; di-ethyl sulfide; di-ethyl amine; ethyl chloride; formaldehyde; di-methyl carbonate; di-methyl ketone; n-alkyl amines, n-alkyl halides, n-alkyl sulfides having n-alkyl groups of comprising the range of from about 3 to about 10 carbon atoms; and mixtures thereof. As used herein, the term xe2x80x9coxygenatexe2x80x9d designates only the organic material used as the feed. The total charge of feed to the reaction zone may contain additional compounds such as diluents.
Preferably, the oxygenate feedstock should be fed to the primary reactor and contacted in the vapor phase in a reaction zone with the selected molecular sieve catalyst at effective process conditions so as to produce the desired olefins, i.e., an effective temperature, pressure, WHSV (Weight Hourly Space Velocity) and, optionally, an effective amount of diluent, correlated to produce olefins. Alternately, the process may be carried out in a liquid or a mixed vapor/liquid phase. When the process is carried out in the liquid phase or a mixed vapor/liquid phase, different conversions and selectivities of feedstock-to-product may result depending upon the catalyst and reaction conditions.
The temperature employed in the primary reaction zone may vary over a wide range depending, at least in part, on the selected catalyst. Although not limited to a particular temperature, best results will be obtained if the process is conducted at temperatures in the range of from about 200xc2x0 C. to about 700xc2x0 C., preferably in the range of from about 250xc2x0 C. to about 600xc2x0 C., and most preferably in the range of from about 300xc2x0 C. to about 500xc2x0 C. Lower temperatures generally result in lower rates of reaction, and the formation of the desired light olefin products may become markedly slow. However, at higher temperatures, the process may not form an optimum amount of light olefin products, and the coking rate may become too high.
When an auxiliary reactor is used, the temperature range may be optimized to more effectively convert C4+ hydrocarbons to light olefins. Although not limited to a particular temperature, best results will be obtained if the process is conducted at temperatures within the range of from about 300xc2x0 C. to about 750xc2x0 C., preferably in the range of from about 450xc2x0 C. to about 700xc2x0 C. Lower temperatures generally result in lower rates of reaction and the formation of desired lower olefins may become markedly slow. However, at higher temperatures, the process may not form an optimum amount of light olefin products and the coking rate may become too high.
Light olefin products will form in both the primary reaction zone and in the optional auxiliary reactorxe2x80x94although not necessarily in optimum amountsxe2x80x94at a wide range of pressures, including but not limited to autogeneous pressures and pressures in the range of from about 0.1 kPa to about 100 MPa. A preferred pressure is in the range of from about 6.9 kPa to about 34 MPa, most preferably in the range of from about 48 kPa to about 0.34 MPa. The foregoing pressures are exclusive of diluent, if any is present, and refer to the partial pressure of the feedstock as it relates to oxygenate compounds and/or mixtures thereof. Pressures outside of the stated ranges may be used and are not excluded from the scope of the invention. Lower and upper extremes of pressure may adversely affect selectivity, conversion, coking rate, and/or reaction rate; however, light olefins such as ethylene still may form.
The process should be continued for a period of time sufficient to produce the desired olefin products. The reaction cycle time may vary from tenths of seconds to a number of hours. The reaction cycle time is largely determined by the reaction temperature, the pressure, the catalyst selected, the weight hourly space velocity, the phase (liquid or vapor), and the selected process design characteristics.
A wide range of weight hourly space velocities (WHSV), defined as weight feed per hour per weight of catalyst, for the feedstock will function in the present invention. The WHSV generally should be in the range of from about 0.01 hrxe2x88x921 to about 5000 hrxe2x88x921, preferably in the range of from about 1.0 hrxe2x88x921 to about 2000 hrxe2x88x921, and most preferably in the range of from about 20 hrxe2x88x921 to about 1000 hrxe2x88x921. The catalyst may contain other materials which act as inerts, fillers, or binders; therefore, the WHSV is calculated on the feed rate of oxygenate and recycle hydrocarbons and weight of inert, filler, and binder free catalyst.
When an auxiliary reactor is used, a wide range of WHSV""s may be used in the secondary conversion reactor. The WHSV generally should be in the range of from about 0.1 hrxe2x88x921 to about 5000 hrxe2x88x921 preferably in the range of from about 1.0 hrxe2x88x921 to about 1000 hrxe2x88x921, and most preferably in the range of from about 1.0 hrxe2x88x921 to about 100 hrxe2x88x921. The catalyst may contain other materials which act as inerts, fillers, or binders; therefore, the WHSV is calculated on the basis of feed rate of hydrocarbon and weight of inert, filler, and binder free catalyst.
One or more diluents may be fed to the reaction zone with the oxygenates, such that the total feed mixture comprises diluent in a range of from about 1 mol % and about 99 mol %. Diluents which may be employed in the process include, but are not necessarily limited to, helium, argon, nitrogen, carbon monoxide, carbon dioxide, hydrogen, water, paraffins, other hydrocarbons (such as methane), aromatic compounds, and mixtures thereof. Preferred diluents are water and nitrogen.
The ethylene and propylene may be separated from the remainder of the product stream, including the heavy hydrocarbon fraction, using any suitable known means, preferably cryogenically. In one embodiment of the invention, the heavy hydrocarbon fraction (or heavy hydrocarbons separated from the heavy hydrocarbon fraction) is recycled back to the primary reactor along with additional oxygenate feed. The recycled heavy hydrocarbon stream may be reintroduced into the primary reactor in numerous ways, including but not limited to, mixing with a regenerated catalyst stream between a regenerator and the primary reactor and before addition of the oxygenated feed, and mixing with the oxygenate feed and the blend of oxygenate and hydrocarbon feed to the primary reactor in either single or multiple locations along the reaction vessel. The same cycle may be repeated until no net C4+ products are produced, or until only a desired amount of C4+ product remains. This embodiment has the advantage that the exothermic conversion of oxygenate to olefins is counteracted to some extent by the endothermic C4+ cracking reactions.
In a second embodiment, the heavy hydrocarbon feed is cycled to a separate auxiliary reactor in which process conditions and catalyst are selected to optimize the production of light olefins from heavy hydrocarbons. Although not intending to limit the invention, preferred catalysts for use in the auxiliary reactor are zeolites, most preferably ZSM-5. By using a separate auxiliary reactor, an optimized oxygenate to olefins catalyst can be used to catalyze the conversion of oxygenates to light olefins in a first reaction zone, a second catalyst can be used to catalyze the conversion of C4+ hydrocarbons to light olefins in a second reaction zone, and both reactors can be optimized in sizing and process condition for the desired reactions to occur.
It may be desirable to provide for C4+ product purge at some point in the cycle to provide a ready source of fuel gas for plant use, C4+ product for sale, or to limit the size of the recycle stream to the primary reactor.
A preferred embodiment for the primary reactor is a circulating fluid bed or riser reactor with continuous regeneration, similar to a modern fluid catalytic cracker. Fixed bed reactors are not practical for use in the primary reactor because oxygenate to olefin conversion is highly exothermic and requires several stages with intercoolers or other cooling devices. The reaction also results in a high pressure drop due to the production of low pressure, low density gas.
Preferred auxiliary reactors include fluid bed, fixed bed, or fired tube reactors. The C4+ cracking process is endothermic; therefore, the reactor should be adapted to supply heat as necessary to maintain a suitable reaction temperature. On-line or periodic regeneration of the catalyst may be provided by any suitable means known in the art.